The present invention relates to a head driving technique for a liquid ejecting apparatus, whereby the voltage of a charge on a charge element is employed to maintain a predetermined bias potential, on the ground terminal side of a pressure generating element, corresponding to a nozzle provided in the head of the liquid ejecting apparatus for ejecting liquid droplets, and whereby, due to the deterioration of the pressure generating element, the elevation of the voltage of the charge on the charge element is prevented.
The liquid ejecting device is used as a record apparatus used with an image record apparatus, a color material ejecting apparatus used for manufacturing a color filter of a liquid crystal display, etc., an electrode material (conductive paste) ejecting apparatus used for electrode formation of an organic EL display, an FED (face light emitting display), etc., a bioorganic substance ejecting apparatus used for biochip manufacturing, a specimen ejecting apparatus as a precision pipet, etc. One form of liquid ejecting device will be discussed by taking an ink jet printer as an example.
Ink jet color printers, used for the ejection from recording heads of several colors of ink, have become popular as output apparatuses for computers, and have been employed for the printing, using multiple colors and tones, of images processed by the computers.
For example, in an ink jet printer using a plurality of piezoelectric elements as driving elements, the piezoelectric elements corresponding a plurality of nozzles of a print head, are selectively driven, and ink droplets are ejected from the nozzles in accordance with the drive voltages applied to the individual piezoelectric elements, thereby the ink droplets are deposited as dots on a printing sheet for printing.
The piezoelectric elements are corresponded to the nozzles for ejecting ink droplets. The ink droplets are ejected based on drive signals supplied by a driver IC (drive wave generator) mounted in the print head.
This type of head driving device is shown in FIG. 5.
In FIG. 5, a head driving device 1 includes piezoelectric elements 2, each of which is corresponded to each of a plurality of nozzles of an ink jet printer; a drive waveform generating circuit 3 for supplying a drive signal to electrodes 2a of each piezoelectric element 2; and current amplifier circuits 4 and switch circuits 5, which are located between the drive waveform generating circuit 3 and each piezoelectric element 2.
While only one piezoelectric element 2 is shown in FIG. 5, since a plurality of nozzles are provided on the head of an ink jet printer, a plurality of piezoelectric elements are provided, one for each of the nozzles.
A drive signal COM, produced by the drive waveform generating circuit 3, is sequentially output, through a shift register, to each of the piezoelectric elements 2.
The piezoelectric elements 2 are provide so as to displace by voltages applied to electrodes 2a and 2b. 
The piezoelectric elements 2a is charged at a level near the intermediate potential (a specific potential between the ground level (GND) and the power source level). And when a discharge is initiated based on the drive signal COM, which has a predetermined voltage waveform and which is supplied by the drive waveform generating circuit 3, ink droplets are ejected by applying pressure on the ink supplied for corresponding nozzles.
The drive waveform generating circuit 3 generates the drive signal COM that is transmitted to the head of the ink jet printer. The drive waveform generating circuit 3 may be located in either the printer main body or the printer head.
The current amplifier circuit 4 includes two drive devices, i.e., first and second transistors 4 and 4b. 
For the first transistor 4a, the collector is connected to a constant voltage power source, the base is connected to a first output terminal of the drive waveform generating circuit 3, and the emitter is connected to the input terminal of the switch circuit 5. With this arrangement, upon the reception of the drive signal COM from the drive waveform generating circuit 3, the first transistor 4a is rendered active and transmits a charge from the constant voltage power source, with a predetermined voltage waveform, through the switching circuit 5 to the piezoelectric element 2.
For the second transistor 4b, the emitter is connected to the input terminal of the switching circuit 5, the base is connected to a second output terminal of the drive waveform generating circuit 3, and the collector is grounded. With this arrangement, upon the reception of a drive signal COM from the drive waveform generating circuit 3, the second transistor 4b discharges the piezoelectric element 2 through the switching circuit 5, with a predetermined voltage waveform.
Based on a control signal, the switching circuit 5 is turned on at the timing whereat a corresponding piezoelectric element 2 is driven, and outputs the drive signal COM to this piezoelectric element 2.
The switching circuit 5 is actually a so-called transmission gate that turns a corresponding piezoelectric element 2 on or off.
When the piezoelectric element 2 is inactive, i.e., when printing is not performed, a charge accumulated on the piezoelectric element 2 will be discharged, due to an insulating resistance, and the voltage dropped, so that ink ejection may be adversely affected.
To resolve this problem, a head driving device is also well known wherein a bias potential, such as the intermediate potential of the drive signal, is maintained on the grounded side of each piezoelectric element. This head driving device has the example configuration shown in FIG. 6.
In addition, there is a piezoelectric element the characteristic of which is improved because a bias potential is maintained, and when such a bias potential is maintained, the absolute value of the potential between the piezoelectric element terminals can be reduced to half at the maximum. Therefore, the withstand voltage of the piezoelectric element can be reduced.
In FIG. 6, a head driving device 6 has substantially the same configuration as the head driving device 1 in FIG. 5, except that a capacitor C1, to which a charge of about +5 V is applied, through a coupling resistor R1, by a constant voltage source Vc1. The capacitor C1 is connected to an electrode 2b of a piezoelectric element 2. The constant voltage source may also be employed as a logic power source.
The capacitor C1, which has a large capacitance, such as 3300 μF, is employed to supply a large current. The coupling resistor R1 is connected to the capacitor C1 to prevent the constant voltage source Vc1 from being adversely affected.
With this arrangement, the voltage of the electrode 2b of the piezoelectric element 2 is maintained at a bias voltage VBS by the voltage charged on the capacitor C1, and the voltage between the electrodes 2a and 2b of the piezoelectric element 2 is reduced. Thus, even when the density at which the piezoelectric elements are provided is high, a discharge between the electrodes of a piezoelectric element can be prevented, or the characteristic of the piezoelectric element can be improved.
However, for this head driving device 6, when deterioration of a piezoelectric element occurs over time, resistance between the terminals is reduced and a leakage current generated. When a leakage current from a constant voltage source Vcc flows through the piezoelectric element 2, this current is applied to and charges the capacitor C1, and also flows to the reference voltage side through the coupling resistor R1.
When the leakage current increases as deterioration of the piezoelectric element 2 advances, and when a leakage current of about 100 mA flows across the coupling resistor R1, at both ends of the coupling resistor R1 the voltage is only about 50 V because the coupling resistor R1 has a resistance of about 500 Ω. As a result, the initial objective, to maintain at the electrode 2b of each piezoelectric element 2 a voltage of about that supplied by Vc1, can not be achieved.
Whereas, since the capacitor C1 has a large capacitance, such as 3300 μF, a capacitor having as low a withstand voltage as possible, such as 6.3 V to 10 V, is employed because of the manufacturing cost.
Therefore, when a leakage current flows to the capacitor C1, a charging voltage that exceeds the withstand voltage will be used to charge the capacitor C1, and this may destroy the capacitor C1.
In order to prevent the damage of the capacitor C1 due to a leakage current from the piezoelectric element 2, recently, as is indicated by an arrow A in FIG. 6, an abnormal voltage detector (not shown) is employed to detect the voltage of the charge on the capacitor C1. When the voltage of the capacitor C1 charge rises until it is equal to or higher than a predetermined voltage, the head driving device 6 is powered off and the operation thereof is halted.
Thus the destruction of the capacitor C1 due to a leakage current from the piezoelectric element 2 can be prevented. However, according to this configuration, regardless of whether deterioration of the piezoelectric element 2 occurs, the head driving device 6 is powered off when a predetermined voltage is exceeded during the charging of the capacitor C1 by the leakage current. Therefore, the piezoelectric device 2 can not be fully utilized up to the expiration of its expected service life.